The invention relates to a method and a device for monitoring insulation and fault current in an electrical alternating current (AC) network, in which the differential current, formed by vectorial addition, between at least two network conductors is ascertained, and a load shutoff is performed whenever the differential current exceeds a certain response value.
In electrical networks, because of defective insulation, fault currents can flow out via ground or via a protective conductor. The voltage drop generated by the fault current can be dangerous to human beings at parts that are touchable but in normal operation are voltage-free (indirect touch). If open, voltage-carrying parts of a current circuit are touched directly, a fault current can flow via human beings and is limited only by the resistance of the human body. Aside from harmed human beings, fault currents can also cause property damage, by influencing electrical systems or by the development of heat energy at the point of the fault. To protect against danger to human beings and property damage from fault currents, in addition to other protective provisions, fault current protection switches (FI protection switches or RCDs) are used. These devices, via a summation current converter, form the vectorial sum of the currents of the network conductors, and, from the outcome of the total differential current, its amount. The total differential current can include AC components and, when DC consumers are connected, such as drives with frequency rectifiers and a DC intermediate circuit, they can also contain DC components. If the total differential current exceeds a certain limit or response value, then the defective current circuit is turned off.
Fault current protective switches, as is generally known to professionals in the field, can be used only with networks of a certain size, because otherwise the natural capacitive leakage currents become greater than the necessary fault current limit value for protecting human beings. The consequences are defective or unintended or unnecessary load shutoffs.
It is therefore an object of the present invention to embody a method of the type defined at the outset such that by distinguishing resistive fault currents from normal capacitive network leakage currents, it can be employed without problems even in relatively large AC networks, and that even in smaller networks, it allows more-accurate monitoring of the fault currents.
This and other objects are attained in accordance with the present invention, wherein the AC component of the differential current is detected as a first network variable; the AC network voltage between at least two network conductors, or between one network conductor and an equipotential bonding conductor or a neutral conductor, is detected as a second network variable; the product of the amplitude of the AC component of the differential current and the cosine of the phase angle "PHgr" between the two network variables detected is ascertained as a measure of the resistive fault current of the network; and the load shutoff is performed whenever the ascertained product exceeds a certain response value.
Such a method is extremely versatile in use and allows both reliable and accurate network monitoring in a way that is not vulnerable to malfunction, in single or multiphase AC networks. This is also particularly true for relatively large AC networks with correspondingly larger natural (capacitive) network leakage currents, in which professionals in the field had until now assumed that reliable fault current or insulation monitoring by means of fault current protection switches and summation current converters was impossible.
In electrical AC networks, consumers that are capable of generating direct leakage and fault currents are used more and more often and actually uncontrollably. These direct currents are caused by electronic elements, such as rectifiers, thyristors, TRIACs or transistors, which are used to convert the alternating voltage into a direct voltage or into an alternating voltage of a different frequency. If insulation faults occur behind these elements, then the fault current includes major DC components. Examples of such devices are primary-clocked switched-mode power supplies in electrical equipment, rectifiers and interrupt-free power supplies or frequency inverters for variable-rpm motor drives, which are all being used increasingly.
It is thus especially advantageous for safety reasons to be able to take into account not only resistive AC components of the network, but also, in accordance with claim 8, to provide differential current detection that is sensitive to universal current, that is, one in which even DC components, which must always be assessed as being resistive, can be taken into account. This makes substantially more versatile and safe use possible, even in the presently typical AC networks that have considerable DC components.
It is advantageous for the differential current, detected with universal current sensitivity, to be split into an AC component and a DC component, the latter always to be addressed as a fault current, as well as to ascertain the resistive AC fault current signal from the AC component and then to add the resistive AC and DC fault currents quadratically. Preferably, the AC component after being filtered out is immediately subjected to frequency weighting for the sake of protecting human beings, an example being low-pass filtration that simulates the frequency dependency of the human body.
According to another aspect of the present invention, two limit or response values of different magnitude are provided, namely a lesser one, of 30 mA, for instance, for a comparison with the ascertained resistive fault current, and a greater one, for instance of 300 mA, for a comparison with the total differential current that has been detected with universal current sensitivity. If the limit value is exceeded, a load or network shutoff is effected. The limit values can be adjustable, and they can also be adapted flexibly to prevailing network conditions.
In extensive networks, until now, it was impossible to use fault current devices with limit values for protecting human beings, if for no other reason because the natural leakage current, which was present because of how the installation is constructed and in many cases was not reducible, was above the limit value of the protection device. In many applications, such as on construction sites with cables that can be damaged during the work, this is highly problematic and even today often leads to accidents. Since in the present method the resistive fault current is ascertained in a targeted way as part of the total differential current and evaluated, for the first time appropriate distinctions and separate monitoring for protecting human beings and protecting equipment are possible. No ways of attaining this in a similar way were known, even though the problem of high leakage currents, which initially made it seem impossible to protect human beings via an FI or fault current protective switch, had already existed ever since there had been electrical distribution networks.
Two different limit or response values are provided, which are also differently frequency-weighted, to make it possible simultaneously to protect both human beings and property. Until now, either only a fault current protection provision with a low limit valve (such as 30 mA) could be used for protecting human beings, or a property protection provision with a higher limit value (such as 300 mA) could be used. In the known passive devices until now, there was also no possibility of frequency weighting using a low-pass filter for protecting human beings and a universal pass filter for protecting property, because these passive devices had to be optimized to the network frequency because of the necessary sensitivity and therefore have only a very narrow bandwidth (approximately 30 Hz to a maximum of one kHz) even in experiments with active fault current protection devices (using auxiliary voltage and electronics, the frequency range had to be limited to approximately 1 kHz at the top by a low-pass filter, since in networks that include harmonics, the harmonics that they generated in the capacitive leakage currents (and not in the resistive fault currents) caused the equipment to trip at the wrong times.
In accordance with another aspect of the present invention in an AC network with an equipotential bonding conductor, enable functional monitoring of the equipotential bonding conductor. This is extremely advantageous for safety reasons, because if there is a fault in the equipotential bonding conductor, the entire network leakage current, or the total differential current detected with universal current sensitivity, can become a fault current that is dangerous to human beings. This dangerous situation can be detected in that in the equipotential bonding conductor, a comparatively high current drop is ascertained, which in the worst case is down to the value of zero. In the normal situation, the total differential current is correspondingly split into the resistance of the protection conductor or equipotential bonding conductor and the resistance (grounding resistance) of an additional grounding means. Thus in the normal situation, proportionality exists between the total differential current and the current in the equipotential bonding conductor.
A load or network shutoff that in the case of a fault has universal polarity, or in other words a total shutoff, is preferred for safety reasons. This is true because this prevents the network from being turned on again even though the network still has insulation faults. Without that provision, an unnecessary, repeated, turn-on and turn-off would take place.
It is also an object of the present invention to create a device suitable for performing the method of the invention. To that end, a device having a differential current sensor that includes at least two network conductors of an AC network, and having a differential current relay, which via a power or load switch performs a load or network shutoff as soon as the differential current exceeds a certain response value, is characterized in that the differential current relay has an electronic phase module, which from the AC component of the differential current which component is ascertained via a high-pass filter, and from the AC network voltage or a comparison voltage derived from it, taking into account the phase angle xcfx86, a fault current signal is ascertained, which represents the AC-dictated resistive fault current of the AC network; and that the phase module is followed by a comparator module, which compares the fault current signal with a lesser response value suitable for protecting human beings and triggers the load switch to trip it if this response value is exceeded.
This device, with the phase module, enables effective realization and performance of the method of the invention. This is true above all in conjunction with the preferred characteristics of the refinement with universal current sensitivity, because as a result, a substantially larger area of potential application is gained, along with markedly greater safety. This is also true for the characteristics of the attendant separate monitoring for protecting human beings (with a lesser resistive fault current) and for protecting property (with a greater total differential current, which also includes the high capacitive leakage currents that rise with the size of a network).
Fault current protection circuits that are based on the principle of magnetic summation current or differential current detection have been known since the 1920s. Only in the 1960s, however, was a response sensitivity attained that made protection of human beings by low response values (10 to 60 mA) fundamentally possible. In Germany, up to the present day, only fault current protection devices independent of network voltage have been described by the standards and employed for protecting persons against direct and indirect touch. Fault current protection devices independent of network voltage detect the differential current of a network with the aid of a magnet core through which the current-carrying network conductors are passed and which has a secondary winding in which a voltage is induced if a differential current occurs. The secondary side is terminated in such a way that the induced voltage can drive a sufficiently high current that then electromagnetically actuates a switch lock, which with switch contacts disconnects the network. The tripping system is electrically and mechanically adapted such that the tripping takes place at a fixed response value (for protecting human beings, typically 30 mA). The relatively low actuation energy available requires very sensitive adaptation between the sensor system and the magnet system, including the mechanics for disconnecting the switch contacts. To prevent the sensitive mechanics from becoming xe2x80x9cmore sluggishxe2x80x9d because of deposits and thus to prevent the response value from being raised to the point that under some circumstances no tripping occurs, the system has to be regularly xe2x80x9cput into motionxe2x80x9d. This is done by actuating a test key, which simulates a differential current and trips the fault current protective switch. In practice, however, fault current protection switches are not tested as required, and thus in the case of a fault the protection provision is often not guaranteed.
Auxiliary-voltage-dependent electronic systems, which do not have this disadvantage, have until now not been permitted, for safety reasons. Up to now, it has been assumed that because of possible failures of electronic components, the protection provision in the event of a fault could not be guaranteed.
With the fault current protection device of the present invention, however, this prejudice is overcome, and this involves an extremely safe, auxiliary-voltage-dependent electronic system that is supplied from the network to be monitored. By various provisions, very high functional reliability is attained, which far exceeds that of conventional devices. The system has constant self-monitoring of function, with a load or network shutoff if an equipment fault occurs. A redundant voltage supply is provided, which remains functional even if some parts fail. A storage capacitor itself assures safe shutoff even if there is a defective voltage supply.
The function of the equipotential bonding conductor can be reliably monitored. This process canxe2x80x94but need notxe2x80x94be done with universal current sensitivity.
The use of a digital interface becomes possible only with the use of electronic circuitry and opens up completely new possibilities. As a result, even completely different safety-relevant measurement variables, known in a different context, can be used as further shutoff criteria. This is true for instance for the insulation resistance of the shut-off network, or for the resistance of the grounding in the fault current protection device. This combination of insulation monitoring, known only from an insulated IT network, with a fault current protection device (which can be used for both grounded and insulated networks) is novel and leads to surprising combination effects. In addition, the novel fault current protection device can now take on the new task of precautionary monitoring, by monitoring instances of where the insulation is becoming worse. As a result, it is now possible to prevent an incipient shutoff by performing early maintenance or troubleshooting. This is true especially because the fault current that indicates only a worsening with respect to property is selected in a targeted way.